How Rare Is Life?

 

by Marcelo Gleiser

Janice Haney Carr/CDC Simple cells, like this river bacteria, began to emerge on Earth only 3.5 billion years ago.

In my post last week on Hawking’s claim that science has shown that God is unnecessary to explain Creation, I made a comment about life:

Furthermore, Hawking also claims that the universe is "just right" for generating living beings like us. Instead, I argue that life is rare (take a look around our solar system neighbors for starters and then look at the history of life on Earth, and all that had to happen for multicellular organisms to thrive) and that complex, intelligent life much rarer still. If anything, intelligent life is a fluke in our universe, the exception and not the rule.

A large number of readers reacted to this, essentially claiming that I had no observational grounds to make this sort of claim. Today, I’d like to revisit the issue, which is not only of great importance — Are we alone? — but also reveals how science works, by careful analysis of available data and the testing of hypotheses.
First, note that my comment was not about life but about complex, intelligent life. There is a huge difference! Simple life may be abundant. Indeed, what we know of the only sample of life — planet Earth — indicates that given the right conditions, life finds its way to emerge. Earth formed some 4.5 billion years ago from the same nebula that gave rise to the sun and all other planets of the solar system. During the first 600 or 700 million years, Earth was a veritable hell, constantly bombarded by the debris left over from the formation of the planets and their moons. Under these conditions, life was impossible. Earth didn’t have a solid crust and its surface would constantly melt due to the energy of the violent impacts.
At around 3.9 billion years ago, things got relatively quiet. Simple chemicals reacted and by 3.5 billion years ago, the first simple life forms were swimming in Earth’s shallow ancient oceans. Some scientists would claim that even by 3.8 billion years ago there were signs of life, but these findings remain controversial. Even if we take the 3.5 billion years ago as a starting point, we can state that life on Earth formed within a few hundred million years, which is quite fast in planetary scales. So simple life may be fairly easy to assemble. What about complex life?
  These were prokaryotes, primitive cells with exposed genetic material. More modern cells, like the eukaryotes we are made of, have the genetic material protected in sacs (the DNA is in a nucleus). The prokaryotes reigned supreme for about two billion years. Only then (numbers are rounded up here) eukaryotic cells appeared. As I discussed in another blog on sponges and creationism, sponge-like multicellular creatures only appeared some 500 to 700 million years ago, give or take a couple of hundred million years.
That is, for about three billion years, life on Earth was only in the form of single-celled organisms.
The transition from unicellular to multicellular life forms remains unclear. Given what we know from natural selection, quite possibly cell colonies found it advantageous to gather together — united we win! — and function as a whole being. Or some ate the other and formed a joint being. However, it’s clear that when we study the history of life on Earth we can’t separate it from the history of Earth itself. As a planet, Earth co-evolved with life. For example, the single-celled blue-green algae that populated Earth’s primitive oceans discovered photosynthesis (accidentally of course), the ability to use the sun’s light to create the energy they needed to survive. With this, they emitted oxygen, a gas that didn’t exist in Earth’s primitive atmosphere for about 2 billion years. It was this surplus of oxygen that allowed for more efficient metabolic pathways which, eventually, led to more complex life forms.
In a very real sense, we are here because our distant unicellular ancestors learned to exhale oxygen.
So, when we ponder on the existence of complex multicellular life in the cosmos we must take into account the conditions that are favorable for complex life to not only form but to survive for long times. It’s not just a question of having liquid water and the right chemicals. (These, of course, are a must.) The planet must have a stable orbit and relatively stable temperatures. In the case of the Earth, the reason why we have four seasons and temperatures that don’t vary like crazy is that we have a large moon. The moon stabilizes the tilt of the Earth (the Earth is like an inclined spinning top with a tilt of 23.5 degrees from the vertical), allowing for stable seasons and liquid water for billions of years. Had the moon been lighter, the Earth’s axis would wobble randomly, oceans could freeze for long periods of time, and complex life would be very hard-pressed to survive.
Earth also has two very important “blankets” that protect life from the nasty and lethal radiation that constantly rains from the skies. Earth’s magnetic field funnels electrically-charged particles coming from the sun into the poles. Sometimes, we see them as auroras at high latitudes. Also, the ozone layer protects us from nasty UV radiation from the sun. Living near a star is no picnic.
Some of these arguments were masterfully developed in the book Rare Earth by Peter Ward and Donald Brownlee published in 2000. There are other arguments that we don’t have the space to address. In my last book A Tear at the Edge of Creation, I updated their arguments, placing them in the context of what we know now.
Finally, the jump from multicellular to complex multicellular and intelligent life is also a fluke. Just think that intelligence emerged only very recently, some 3.5 billion years after life emerged. It’s common to think that intelligence and even complexity is a natural consequence of evolution, that is, if there is life give it enough time and it will turn smart.
That’s not what natural selection tells us. All life cares about is being well-adapted to the changing planetary environment. (And by the way, that’s only possible because genetic reproduction allows for mutations to occur. Had it been perfect, life would fail.) Think of the dumb and fearsome dinosaurs that were here happy for 150 million years. No intelligence there, just good adaptation.
Surely, given the enormous number of other planets and moons out there in the galaxy (and in the hundreds of billions of other galaxies!) and the fact that the laws of chemistry and physics apply throughout the cosmos, life should not be an Earthly phenomenon. But whatever life forms exist out there, given what we know of how life evolved here on Earth and the many conditions that come into play in order to sustain complex life, it’s a huge jump to assume that complex life is equally widespread. Intelligent life even more so. I could also use the fact that we haven’t been visited by aliens and explore Fermi’s paradox, but I’ll leave that for another day. But even if there are other intelligent life forms in the galaxy, and we can’t either prove or rule that out with the evidence at hand, the reality is that for practical purposes we are alone and will probably be for the foreseeable future.
The mess we make is the mess we take.